Hypertension
It is generally accepted that high blood pressure (HBP, also called hypertension) is bad, but most people don't know why, and what the term really means. In fact, all humans have high blood pressure some of the time, and we wouldn't be able to function if we didn't (such as during exercise). High blood pressure is only of concern when it persists for long periods of time or is extremely high over a very short (hours) period of time. Its adverse effects usually take many years to develop. Clinically important HBP is very common. According to official government figures it affects 50 million people in the United States.
While everyone has high blood pressure some of the time, many people live their entire lives with moderately high blood pressure and never know it until it is notice on a routine visit to the doctor. Unfortunately, not all people are so lucky. In these people, high blood pressure significantly increases the risk of a number of serious events, mainly strokes and heart attacks.
More specifically, the damage caused by high blood pressure is of three general sorts. The first is the one everyone thinks of—bursting a blood vessel. While this is dramatic and disastrous when it happens, it's actually the least common of the three problems. It occurs most frequently in the blood vessels of the brain, where the smaller arteries may develop a weak spot, called an aneurysm. This is an area where the wall is thinner than normal and a bulge develops. When there is a sudden surge of pressure the aneurysm may burst, resulting in bleeding into the tissues. If this occurs in the brain, it is called a stroke. In contrast, if this happens to the aorta (the main blood vessel in the body), it is called a ruptured aortic aneurysm. Both of these events can lead to permanent damage and death.
The second adverse consequence of high blood pressure is that it accelerates the deposition of cholesterol in the arteries forming a blockage (atheroma). This problem, too, takes many years to develop, and it is very difficult to detect until it causes a major blockage. The most important sites to be affected are the heart, where the blockage can cause angina and heart attacks; the brain, where it causes strokes; the kidneys, where it causes renal failure (and can also make the blood pressure go even higher); and the legs, where it causes a condition known as intermittent claudication, which means pain during walking and may even lead to losing a limb.
Third, high blood pressure puts a strain on the heart: Because it has to work harder than normal to pump blood against a higher pressure, the heart muscle enlarges, just as any other muscle does when it is used excessively. Over a long period of time, the high blood pressure can lead to congestive heart failure, the most frequent cause for hospitalization in the United States.
Whatever the underlying cause, when the blood pressure reaches a certain level for a sufficient length of time it sets off a vicious cycle of damage to the heart, brain, and kidneys, resulting in further elevation of the pressure.
Classification of hypertension by its severity is somewhat arbitrary because there's no precise level of pressure above which it suddenly becomes dangerous. Historically, blood pressure has been primarily classified according to the height of the diastolic pressure. Someone whose diastolic pressure runs between 90 and 95 mm Hg may be regarded as having borderline hypertension, and when it's between 95 and 110 mm Hg it's moderate, and at any higher levels it's severe. Recent data suggests that the systolic pressure is as, and maybe more important than, diastolic blood pressure in determining the patient's risk for serious adverse events. Systolic hypertension is mainly seen in people over the age of 65 and is characterized by a high systolic, but normal diastolic, pressure (a reading of 170/80 mm Hg would be typical). It's caused by an age-related loss of elasticity of the major arteries. Another form of HBP, Labile hypertension, is a commonly used term for describing people whose pressure is unusually labile or variable. The most dangerous type of HBP is called malignant hypertension, or high blood pressure with evidence on physical exam that this pressure causing an acute deleterious affecting on vital organ function. Malignant hypertension is regarded as an emergency requiring immediate treatment in a hospital. Not surprisingly, if untreated, malignant hypertension can be rapidly fatal. Because more people are treated nowadays than before, malignant hypertension is not common.
The objective of treatment is not simply to lower the blood pressure, but to prevent its consequences, such as strokes and heart attacks. According to the American Heart Association high blood pressure is present in 50,000,000 Americans (Defined as systolic pressure 140 mm Hg or greater, and/or diastolic pressure 90 mm Hg or greater, or taking antihypertensive medication). Of those with HBP, 31.6 percent are unaware they have it; 27.4 percent are on medication and have it controlled; 26.2 percent are on medication but don't have their HBP under control; and 14.8 percent aren't on medication. In most cases, high blood pressure can be controlled with one or a combination of oral drugs. Of those patients that take medication to control HBP, many suffer from debilitating side effects of these drugs such as heart arrhythmias, inability to exercise or do normal activities of daily living and impotence.
High blood pressure is usually treated with drugs. Several proposals have been made to treat blood pressure with electrical stimulus applied to various parts of the body.
Group 1: Controlling Hypertension With Highly Invasive Implantable Nerve Stimulation
Several proposals have been made to treat moderately elevated blood pressure using highly invasive methods such as a vagal (part of the vagus nerve) nerve stimulation, spinal cord stimulation and deep brain stimulation. It has been known in the past that one can stimulate the vagal nerves by invasively dissecting the major nerve bundle and placing a spiral or enveloping nerve-type cuff around the nerve bundle. The nerve fibers are then directly stimulated by electrical field to achieve reduction in epilepsy, heart rate slowing, and potential blood pressure changes.
Currently, only nerve cuff-type electrodes or impalement-type electrodes are used for nerve stimulation, other than in the spinal cord. These types of electrodes can potentially cause irreversible nerve damage due to swelling or direct mechanical damage of the nerve. The placement of these electrodes either around the nerve bundle or into the neural perineum also poses a significant risk. The electrode placement is usually performed through very invasive surgery which in and of itself produces a high risk to nerve damage.
Terry, Treating Refractory Hypertension By Nerve Stimulation, U.S. Pat. No. 5,707,400 proposes implantation of an electrical coil or cuff around the vagus nerve, which runs superficially through the neck, and stimulation of the vagus nerve to lower high blood pressure.
Kieval, Devices and methods for cardiovascular reflex control, U.S. Pat. No. 6,522,926 and several other patents to the same author describe devices, systems and methods by which the blood pressure is reduced by activating baroreceptors. The baroreceptors are the biological sensors in the wall of the carotid artery that indicate to the brain an abrupt rise or fall of blood pressure by responding to the stretch of the arterial wall. In response to baroreceptor stimulation, the brain reduces the pumping of the heart with the consequential moderation of blood pressure. This phenomenon is known as the body's “baroreflex”.
Obel, Implantable electrical nerve stimulator/pacemaker with ischemia for decreasing cardiac workload, U.S. Pat. No. 5,199,428 describes a method and apparatus for stimulating the right and/or left carotid sinus nerves or the right stellate ganglion or the epidural space of the spine electrical pulses in response to detected myocardial ischemia to decrease cardiac workload as a method to protect the myocardium.
The methods described above are potent and are capable of, at least temporarily, reducing blood pressure in a patient. They do not meet the objectives of the invention because they are highly invasive and have potentially debilitating or life threatening side effects. In general, it may be said that these methods attempt to regulate blood pressure by directly disturbing the vital parts of the central nervous system such as brain, spinal cord, vagus nerve and carotid sinus nerves. The potential side effects of such a device, including nerve damage, paralysis and death make the use of these methods unlikely except in the most severe cases where the high risk can be justified.
Group 2: Non-Invasive Nerve Stimulation to Treat Hypertension
All the devices that stimulate nerves non-invasively stem from traditional Chinese medicine use of acupuncture to treat essential hypertension (or HBP without an identifiable cause). Acupuncture has also been investigated for use in patients with essential hypertension in the Western countries. In spontaneously hypertensive rats, acupuncture-like electrical stimulation of thinly myelinated (Group III) somatic afferents activates central endorphin (natural opiate) pathways that elicit long-lasting decreases in sympathetic nerve activity (SNA) and blood pressure. Based on this data, the following major hypotheses can be argued: electroacupuncture (acupuncture with electric current) produces a long-lasting reduction in SNA, thereby providing a safe and effective complementary treatment of human hypertension. Pomeranz, et al, Electrotherapy Acupuncture Apparatus and Method, U.S. Pat. No. 4,566,064 describes an electroacupuncture device and mentions blood pressure as an indication for electroacupuncture.
Nevertheless, even if effective, acupuncture has little practical use. It requires frequent visits to the doctor for prolonged sessions that require commitment of time often impossible for working patients. Puncture of skin and manipulation of needles can be painful. It is also expensive. Several inventors proposed devices, based on acupuncture, that the patient could wear and use at home.
Zhu, Blood Pressure Depressor, U.S. Pat. No. 5,891,181 proposes electrical stimulation of nerves in the ear lobe to lower blood pressure.
Gruzdowich, Method of blood pressure moderation U.S. Pat. No. 6,393,324 describes a HBP control device in the form of a watch-like housing attachable to the human wrist by an adjustable attachment band. The device uses non-invasive nerve stimulation whereby electricity is passed through two electrodes to stimulate nerves located on the palm side of the wrist. The treatment provided by the device is based on the acupuncture of the P6 point, pericardium 6 point, or the master point of the pericardium meridian (sometimes referred to as the vascular meridian).
Non-invasive devices based on these inventions are available on the market and can be purchased without a prescription for reasonable price. These non-invasive devices are generally safe, require no surgery but are not very effective. Position of the electrodes over the particular point of a muscle or specific nerve is approximate and is easily disrupted. Skin has high resistance and high stimulation voltage is needed to achieve the therapeutic effect. Often pain from the electric stimulation is felt before blood pressure is reduced. Patients are required to take care of the device, turn it on and off, replace batteries and ensure that it is always placed correctly.
Group 3: Transvenous Nerve Stimulation to Control Heart Rate
Hill, U.S. Pat. No. 6,006,134, describes a method and device for electronically controlling the beating of a heart during cardiac surgery. Slowing the heart rate can result in a reduction of blood pressure. Hill proposed using venous electrical stimulation of nerve fibers, describes an electro-stimulation device that includes a pair of electrodes for connection to at least one location in the body that affects or regulates the heartbeat. The invention is embodied in an external or implantable device which employs electrodes located on transvenous leads located in veins adjacent nerve fibers to be stimulated. The transvenous placement of stimulation leads reduces the invasiveness of the procedure.
Hill does not teach treatment of hypertension with an implantable device. Hill also does not point towards transvenous stimulation of peripheral nerves in an arm or a leg. Instead Hill proposes to simulate nerves of the heart and the vagus nerve, and admits that the stimulation may not only slow down but also stop the heart. Hill describes the electrodes that are placed on the catheter in an internal jugular vein several centimeters from the vagal nerve bundle. The amount of applied current for this type of stimulation will be too high to use in conscious patients since it may cause pain and muscle twitching. It is not therefore surprising that Hill proposed the transvenous stimulation during surgery, where patient is unconscious and constantly under observation.